This invention relates to expanded polystyrene foam products or other open-cell foam products which are useful in the preparation of lightweight building materials, such as insulation boards and the like, and more particularly to expanded polystyrene foam products having open cells or spaced beads of polystyrene wherein the material is impregnated with paraffin, candelilla, montan and other types and blends of waxes to increase the product's density and both its compressive and tensile strengths thereby rendering it resilient, waterproof, and weatherproof at relatively low cost.